An alumina support was modified by fluorine via impregnation to investigate the effect of fluoride content on the reactivity of Ni–Mo/Al2O3 catalyst. The catalyst was characterized by X-ray diffraction, N2 adsorption–desorption (Brunauer–Emmett–Teller) isotherms, temperature-programmed desorption of ammonia, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. Sulfur etherification performance of the catalyst was studied using a fixed-bed reactor. The results show that increasing fluoride content increases the pore volume and pore size but reduces the specific surface area. In addition, the degree of sulfidation of Ni first increases and then decreases. The amounts of strong acid and total acid also increase with increasing fluoride content. Performance evaluation of the catalyst reveals that the fluoride content has a minor effect on the thioetherification performance of the catalyst; however, an optimum fluoride content, which was determined to be 0.2%, can ensure lower olefin saturation and an efficient diene selective hydrogenation.